Draft: keyboard: Get default input sources from gnome-desktop

Right now, we figure out the default input sources ourselves, based on the current locale and layout information coming from localed.

This logic needs to be duplicated in several components, so its now provided by gnome-desktop.

This commit changes it over to use gnome-desktop APIs.

The same time if leverages a gnome-desktop API to fix a bug where cyrillic layouts were getting added without a latin counterpart.

Fixes #194